Echinoderms are a diverse group of marine animals, encompassing familiar creatures like starfish, sea urchins, sea cucumbers, brittle stars, and sea lilies. They are found in all of the world’s oceans, from shallow intertidal zones to the deepest abyssal plains. Despite their varied appearances and lifestyles, all echinoderms share fundamental characteristics that distinguish them.
Distinct Body Plan
Adult echinoderms are defined by their pentaradial symmetry, meaning their bodies are organized around a central axis in five, or multiples of five, equal parts. This radial arrangement is evident in the five arms of a typical starfish or the five-part structure of a sea urchin’s shell. This differs from the bilateral symmetry observed in most other animal phyla, where a body can be divided into two mirror-image halves.
Despite their adult radial symmetry, echinoderms begin their lives with bilateral symmetry during their larval stages. As the larva undergoes metamorphosis, its body reorganizes, and the left side of the larval body develops into the adult’s oral surface, while the right side becomes the aboral surface. This developmental shift highlights their evolutionary lineage from bilaterally symmetrical ancestors.
Unique Internal System
Echinoderms possess a specialized water vascular system, a network of fluid-filled canals. This system is crucial for many life functions, including movement, feeding, and gas exchange. Water enters this system through a sieve-like plate called the madreporite, typically located on the animal’s upper surface.
From the madreporite, water flows through a stone canal to a ring canal that encircles the mouth. Five radial canals extend from the ring canal into each arm or body section, branching into numerous small, muscular appendages known as tube feet. These tube feet operate through hydrostatic pressure; when muscles contract, forcing water into them, they extend and can attach to surfaces via suction cups, enabling the animal to move, grip prey, or hold onto the seabed.
Skeletal Structure
Echinoderms also have an internal skeleton, or endoskeleton, embedded within their body wall. This skeleton is composed of numerous small, calcareous plates called ossicles, made of calcium carbonate. The term “Echinodermata” itself translates to “spiny skin,” referencing the often rough or spiny texture created by these ossicles.
These ossicles can be tightly fused, forming a rigid structure like the “test” of a sea urchin, or they can be more loosely articulated, allowing flexibility as seen in the arms of starfish. The endoskeleton provides support and protection for the animal’s soft tissues. Unlike the external skeletons of insects or the internal bones of vertebrates, the echinoderm endoskeleton grows with the animal and is covered by a thin layer of epidermis.
Additional Defining Traits
Echinoderms are exclusively marine animals. Their presence spans all ocean depths, from coastal areas to deep-sea trenches.
Many echinoderms exhibit regenerative capabilities, allowing them to regrow lost body parts, such as arms or spines. Some species can even regenerate an entire organism from a single arm, demonstrating a high degree of tissue and organ repair. This ability is a significant survival mechanism against predation or injury.
Some echinoderms, particularly sea stars and sea urchins, possess specialized pincer-like structures called pedicellariae. These small, movable appendages have tiny jaws and serve various functions, including keeping the body surface clean of debris and small organisms, defending against predators, and aiding in food capture. These unique traits collectively define the phylum Echinodermata, highlighting their distinct evolutionary path within the animal kingdom.